size of decompression bubbles 2

[anton115]

Hi Dr Deco,

I am reposting a question from a previous thread titled "size of decompression bubbles" hoping to get your thoughts.

Thanks Dr Deco,
I was wondering if you had and information on the upper size limits of bubbles produced on dives to say 30-40 meters when diving to the no deco limit. The reason I ask is that looking at DAN Project's Project safe dive It claims that 37.4% of divers tend to get IV bubbles on a single dive but on repetitive dives 85% of divers get bubbles with 67% of divers having high grade bubbles. It also states that bubbles are more common and of a higher grade in dives deeper than 30m.
Now it seems general consensus for there to be no need for PFO screening and PFOs are present in 25-30% of the population. However 4-5% of people have a PFO with a large right to left shunt at rest. Members of this population would have a signifiant number of bubbles passing to the arterial side after a repetitive dive to 30m which is common. Even though neurological DCS is much more common in this group it seems that most cases of decompression with arterialisation of many bubbles in this group produces no symptoms.

I was wondering if decompression bubbles, when arterialised, tend to be small enough to simply pass through the arterioles and capillaries and is it a rare occurrence when these bubbles coalesce to form a large bubble the cerebral DCS manifests or are the decompression bubbles ofter large enough to produce damage but are ofter just causing silent lesions hence there being no symptoms?

Thanks Anton

 

[DocVikingo]

I was wondering if decompression bubbles, when arterialised, tend to be small enough to simply pass through the arterioles and capillaries .... and is it a rare occurrence when these bubbles coalesce to form a large bubble the cerebral DCS manifests or are the decompression bubbles ofter large enough to produce damage but are ofter just causing silent lesions hence there being no symptoms?Thanks Anton

Hi Anton,

I'm having trouble understanding your sentence after my ..... Are the grammar, syntax & spelling in fact as you intended?

Thanks,

DocVikingo

 

[anton115]

Hi DocVikingo,

I will rephrase my question:

I wanted to know what the upper size limit would be of bubbles typically produced on a dive to the RDP No Deco limits? I took the example of a dive to 30m for 20min as this is common for a recreational dive.

The reason I ask is that it is general consensus that PFOs are not a contraindication to diving but there is a large variation in shunting by PFOs. about 30%of people have a PFO but about 5% of people have a PFO with large right to left shunt at rest. A PFO with a large shunt is functionally the same as an ASD which is a contraindication to diving.

People diving with these large right to left shunting PFOs will be shunting many bubbles when they decompress from a dive deemed safe for the general population but still likely to produce venous bubbles. I extend my question to ask, is the size of the bubbles in the venous system on a dive to no deco limits small so if they enter arteries they simply pass through the microvasculature with no harm, or are they large enough to be causing subtle damage?

Thanks
Anton

 

[Glenn08]

While you are theorizing = blood is a fast tissue. DCIEM did a doppler study years ago that detected micro-bubbles in venous circulation. Part of the theory was that the bubbles formed in the veins because blood pressure is lower than in the arteries. The capillaries in the lungs may trap the bubbles. This also makes bounce dives seem more dangerous. I don't know what follow-up has happened on the study, or if it has been incorporated into decompression theory.

 

[Dr Deco]

Hello anton115:

Bubble Size

When divingto the NDL, or any time and depth for that matter, the number of bubbles producedwill depend on the gas loads and will vary from diver to diver. Some divers are prone to bubble formation andthe reason is not yet known. I do not have any data on dives to differing NDLs.

In myearliest studies in the early 1970s, I looked at the question of bubble formation[in rats – easy to study] following decompression.

(1)MR Powell. Leg pain and gas bubbles inthe rat following decompression from pressure: monitoring by ultrasound: Aerospace Med., 43, 168-172 (1972).
(2)MR Powell. Gas phase separationfollowing decompression in asymptomatic rats: visual and ultrasoundmonitoring: Aerospace Med., 43,1240-1244 (1972).

It was clearthat early post decompression the bubbles were tiny [as viewed by a hand lensthrough the thin-walled vena cava of anesthetized rats] and few in number. As the minutes passed, the number and size ofthe bubbles increased. Therefore, it isnot possible to give a specific answer to bubble size and number as it dependson gas loads and time post decompression. The bubbles will reach a maximum and then disappear. Should the worst occur, and a large number ofbubbles appear [usually from considerable activity underwater promoting tissue gasloading], arterialization can happen and the diver could die. Such an event was chronicled in “The FinalDive” about the father and son dive team, Chris and Chrissy Rouse.

High Bubble Grades

The Grades in the DAN Safe Dive project seemhigh to me, but this could vary depending on a degree of subjectivity and theactivity level of the test subjects. It isclear that these high Grades did not result in many cases of DCS; this makes mesuspicious about the actual grades. However,they have the data, so I have little to contest.

Arterialization and DCS

Experience overa couple of decades has shown that arterial-side bubbles do not generallyproduce DCS problems. Bubbles have beenstudied in anesthetized animals, and it appears that atrial pressure drivesthem through the brain capillaries. Thisquestion have also been studied in surgical patients [e.g., in open heartsurgeries]. It appears that bubbles movethrough capillaries unless a large number are present in a given vessel. They then can coalesce and produce a verylong bubble, and apparently arterial pressure is then insufficient to push thenow-long bubble through. Long bubbles have a high “adhesion pressure.”

There is anextra advantage in the circulatory system of the brain. There is an abundance of collateralflow. Several capillary beds supply anygiven volume. This probably protects thebrain from small blood clots that could result in small strokes. There actually are individuals with heartconditions that result in a few miniclots every minute. It is actually somewhat “spooky” to hear the tinyclots in the carotid artery with a Doppler blood flow meter.

Quite frankly,if CNS DCS occurred with any frequency, you can be sure that over the pastcentury, the dive limits would be reduced to prevent this problem. Thus, whatever one might imagine is occurringthat would lead to serious DCS, it does not occur.

PFO Testing

Numerous studieswith NMR have shown that the brains of divers show basically no more lesionsthan the non-diving population. Again,if a problem was spotted, something would be changed. Nature has spoken.

I would agreethat the presence of bubbles and the non-minimal frequency of a resting PFO makeone suspicious. There certainly is reason to wonder but not one to worry. A PFO test is relatively benign but not one absolutelyfree of problems. This coupled with thecost eliminates a blanket recommendation to check all divers for thisdefect. Now, there are occasions whereprofessional divers have repeat episodes of CNS DCS and repair [closure] of thePFO has been recommended and performed. Thisis not the case with recreational divers.

Dr Deco :doctor:

 

[anton115]

Hi Dr Deco,

Thanks for your very thorough response. My interest in this partly stems from being a diver who recently found he had a PFO with a large shunt and has been given different advice from different doctors. cardiologist with an interest in DCS generally say don't dive and dive doctors generally saying to continue diving. (i am not keen on closure)
My interest also comes from the fact that I am a surgeon in training who does a lot of laparoscopy. There is documented risk of air embolism from this although research is currently limited. Although we do not screen for cardiac shunt in a pre op assessment, if one is known about we go for an open procedure rather than a laparoscopic one. The decision seems to be based less on evidence based medicine (as there is little) but more on a medico legal stand point as there is not enough evidence.
I am interested by your point that multiple capillary beds supply a given volume. It was my understanding that surface arterioles had a good collateral flow (leptomeningeal) but the terminal arterioles supplying the cortex do not and micro occlusion of these results in mircroinfarction. This is what is thought to be the explanation of cognitive deficits seen after operations involving cardiac bypass caused by air micro emboli. This can not be seen by MRI but has been seen on histopathology. Many of my cardiologist colleagues believe this happens to divers with shunts where as the hyperbaric physicians disagree. Any thoughts?